The production of solid polymer particles, which are suitable for pelletization and use in subsequent manufacturing processes, may be accomplished by any one of many known polymerization techniques. In such processes, a slurry stream may be formed which consists of the particulate polymer suspended in a liquid hydrocarbon diluent, which acts as the polymerization reaction medium. This diluent may be an inert solvent or a monomer or comonomer. As an example, when ethylene is polymerized in a hydrocarbon or aqueous hydrocarbon diluent such as isobutane under controlled temperature and pressure conditions, a slurry of polymer solids and diluent is formed. This type of process is known as particle form polymerization. One drawback of this kind of process, or any process in which a polymer is prepared in solution and subsequently precipitated to form a slurry, is that the solid polymer must be separated from the liquid portion of the slurry. This liquid portion may include any suitable solvent(s) utilized in the particular polymerization process (typically these are C.sub.3 -C.sub.8 hydrocarbons), and unpolymerized monomer and/or comonomer, all of which will be referred to hereinafter collectively as "hydrocarbon". The term "hydrocarbon" is not intended to include the polymer itself.
One known method for carrying out the liquid-solid separation is to flash the hydrocarbon into a vapor by reducing the pressure on the slurry. This may be accomplished in a suitable flash tank or series of flash tanks. The residual polymer from the flash tank, which contains additional hydrocarbon, is then dried in a fluid bed dryer prior to extrusion and/or pelletization thereof. The gas used in the fluid bed dryer, which may be heated air, nitrogen or any suitable inert gas, vaporizes additional hydrocarbon and removes it from the polymer particulate. The hydrocarbon-containing drying gas is then typically vented to the atmosphere.
The known processes for the removal of hydrocarbon from polymer slurries may not remove a sufficient quantity of the hydrocarbons from the polymer and, thus, the hydrocarbon emissions from the fluid bed dryer may be undesirably high. With the current trend toward environmental awareness and stricter emission standards, there exists a need for both methods and apparatus which are capable of reducing the emission of hydrocarbons from polymer production systems.